Asystole detection and response in an implantable cardioverter defibrillator

ABSTRACT

An implantable cardioverter defibrillator (ICD) receives a cardiac electrical signal by a sensing circuit while operating in a sensing without pacing mode and detects asystole based on the cardiac electrical signal. The ICD determines, in response to detecting the asystole, if asystole backup pacing is enabled, and automatically switches to a temporary pacing mode in response to the asystole backup pacing being enabled. Other examples of detecting asystole and providing a response to detecting asystole by the ICD are described herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/142,074, filed Apr. 29, 2016, entitled “ASYSTOLE DETECTION ANDRESPONSE IN AN IMPLANTABLE CARDIOVERTER DEFIBRILLATOR,” which claims thebenefit of the filing date of provisional U.S. Patent Application No.62/328,803, filed Apr. 28, 2016, entitled “ASYSTOLE DETECTION ANDRESPONSE IN AN IMPLANTABLE CARDIOVERTER DEFIBRILLATOR,” the content ofboth of which is incorporated by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to an implantable cardioverter defibrillator and method fordetecting and responding to cardiac asystole.

BACKGROUND

A variety of implantable medical devices (IMDs) for delivering atherapy, monitoring a physiological condition of a patient or acombination thereof have been clinically implanted or proposed forclinical implantation in patients. Some IMDs may employ one or moreelongated electrical leads carrying stimulation electrodes, senseelectrodes, and/or other sensors. IMDs may deliver therapy to or monitorconditions of a variety of organs, nerves, muscle or tissue, such as theheart, brain, stomach, spinal cord, pelvic floor, or the like.Implantable medical leads may be configured to position electrodes orother sensors at desired locations for delivery of electricalstimulation or sensing of physiological conditions. For example,electrodes or sensors may be carried along a distal portion of a leadthat is extended subcutaneously, submuscularly, or transvenously. Aproximal portion of the lead may be coupled to an implantable medicaldevice housing, which contains circuitry such as signal generationcircuitry and/or sensing circuitry.

Some IMDs, such as cardiac pacemakers or implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to theheart of the patient via electrodes carried by one or more implantableleads and/or the housing of the pacemaker or ICD. The leads may betransvenous, e.g., advanced into the heart through one or more veins toposition endocardial electrodes in intimate contact with the hearttissue. Other leads may be non-transvenous leads implanted outside theheart, e.g., implanted epicardially, pericardially, or subcutaneously.The electrodes are used to deliver electrical pulses to the heart toaddress abnormal cardiac rhythms.

IMDs capable of delivering electrical pulses for treating abnormalcardiac rhythms typically sense signals representative of intrinsicdepolarizations of the heart and analyze the sensed signals to identifythe abnormal rhythms. Upon detection of an abnormal rhythm, the devicemay deliver an appropriate electrical stimulation therapy to restore amore normal rhythm.

SUMMARY

In general, the disclosure is directed to techniques for detecting along ventricular pause or asystole and providing a response to thedetected asystole by an ICD. An ICD operating according to thetechniques disclosed herein operates in a sensing without pacing modeduring which cardiac electrical signals are sensed but no pacing pulsesare scheduled or delivered. The ICD may automatically switch to a pacingmode in response to detecting asystole during the sensing without pacingmode when automatic switching to a pacing mode is enabled. If theautomatic switching is not enabled, the response to the asystoledetection includes storing asystole episode data. If the automaticswitching is enabled, the ICD may switch to a temporary pacing mode insome instances and to a permanent pacing mode in other instances. If theICD automatically switches to the temporary pacing mode, the ICD returnsto the sensing without pacing mode after a temporary pacing modetermination condition is satisfied.

In one example, the disclosure provides an ICD system comprising asensing circuit configured to receive a cardiac electrical signal via asensing electrode vector and sense cardiac events from the cardiacelectrical signal, a therapy delivery circuit configured to deliverelectrical pacing pulses to a patient's heart via a pacing electrodevector, and a control circuit coupled to the sensing circuit and thetherapy delivery circuit and configured to automatically switch betweena sensing without pacing mode and a temporary pacing mode. The controlcircuit is configured to detect asystole based on the cardiac electricalsignal while operating in the sensing without pacing mode; in responseto detecting the asystole, determine if asystole backup pacing isenabled, and automatically switch to the temporary pacing mode inresponse to the asystole backup pacing being enabled.

In another example, the disclosure provides a method performed by an ICDsystem. The method includes receiving a cardiac electrical signal by asensing circuit of the ICD; detecting asystole by a control circuit ofthe ICD based on the cardiac electrical signal while operating in asensing without pacing mode; in response to detecting the asystole,determining by the control module if asystole backup pacing is enabled,and automatically switching to a temporary pacing mode in response tothe asystole backup pacing being enabled.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an ICD system, cause the system toreceive a cardiac electrical signal by a sensing circuit of an ICD;detect asystole based on the cardiac electrical signal while operatingin a sensing without pacing mode; in response to detecting the asystole,determine if asystole backup pacing is enabled, and automatically switchto a temporary pacing mode in response to the asystole backup pacingbeing enabled.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with an ICDsystem in a different implant configuration than the arrangement shownin FIGS. 1A-1B.

FIG. 3 is a conceptual diagram illustrating a distal portion of anotherexample of the extra-cardiovascular lead of FIGS. 1A-2C.

FIG. 4 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 5A and FIG. 5B are diagrams of a cardiac electrical signal and acorresponding cardiac event timeline depicting R-wave sensed eventsignals and pacing pulses generated by an ICD.

FIG. 6 is a flow chart of a method for controlling cardiac pacing by anICD to provide backup pacing in response to detecting asystole.

FIG. 7 is a timing diagram of events during a pacing mode according toone example.

FIG. 8 is another timing diagram of events during a pacing modeaccording to another example.

FIG. 9 is a flow chart of a method for controlling a response todetecting asystole by an ICD according to another example.

FIG. 10 is a flow chart of a method for controlling automatic switchingto a temporary asystole backup pacing mode by an ICD according to yetanother example.

FIG. 11 is a conceptual diagram of an ICD system that may performasystole detection and provide an asystole response according to anotherexample.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for detecting asystoleand providing a response in an ICD. The techniques disclosed herein areused to control the operating mode of an ICD for detecting andresponding to forms of bradycardia, such as a long ventricular pause orasystole. ICD patients often do not require chronic bradycardia pacingand frequent or prolonged single chamber ventricular pacing is generallyundesirable. Some ICD patients, however, may experience asystole or longventricular pauses as their disease progresses. The disclosed techniquesmay be implemented in an ICD to minimize cardiac pacing and promoteintrinsically conducted heart rhythms in patient's receiving an ICDwhile still providing backup support for patients that experience longventricular pauses or asystole or develop a need for bradycardia pacingafter implantation of the ICD system. As disclosed herein, an ICDoperates in a sensing without pacing mode and only switches to abradycardia pacing mode when asystole or a long ventricular pause isdetected following an intrinsic R-wave and automatic switching to apacing mode is enabled.

The sensing without pacing mode and the temporary and permanent pacingmodes described herein refer to operating modes for providing a responseto bradycardia. It is recognized that tachyarrhythmia operating modesfor detecting and responding to tachyarrhythmia may be operating inparallel to the bradycardia operating modes such that, for example,during the bradycardia sensing without pacing mode, bradycardia andasystole backup pacing pulses are not scheduled or delivered but pacingpulses may be delivered as part of a tachyarrhythmia therapy response todetecting tachycardia or fibrillation, including anti-tachycardia pacing(ATP) and/or post-shock pacing. Pacing pulses delivered in response to afast ventricular rhythm or following a pacing shock delivered toterminate a tachyarrhythmia, however, are controlled according toprogrammed tachyarrhythmia operating mode and control parameters and arenot controlled by a bradycardia operating mode.

As used herein, the term “sensing without pacing mode” refers to anoperating mode that includes sensing of a cardiac electrical signal anddetection of asystole and does not include scheduling or delivering anycardiac pacing pulses. However, it is recognized that a tachyarrhythmiatherapy response may occur during the sensing without pacing mode whichmay include anti-tachycardia pacing or post-shock pacing, initiated dueto a fast heart rate or delivering a cardioversion/defibrillation shock.During the sensing without pacing mode described herein, a slow heartrate, e.g., less than a programmed bradycardia lower pacing rate doesnot result in pacing pulse scheduling or delivery.

FIGS. 1A and 1B are conceptual diagrams of one example of an ICD systemin which the techniques disclosed herein may be implemented. FIG. 1A isa front view of ICD system 10 implanted within patient 12. FIG. 1B is aside view of ICD system 10 implanted within patient 12. ICD system 10includes an ICD 14 connected to an extra-cardiovascular electricalstimulation and sensing lead 16. FIGS. 1A and 1B are described in thecontext of an ICD system 10 capable of providing defibrillation and/orcardioversion shocks and pacing pulses.

As used herein, the term “extra-cardiovascular” refers to a positionoutside the blood vessels, heart, and pericardium surrounding the heartof a patient. Implantable electrodes carried by extra-cardiovascularleads may be positioned extra-thoracically (outside the ribcage andsternum) or intra-thoracically (beneath the ribcage or sternum) butgenerally not in intimate contact with myocardial tissue.

Extra-cardiovascular pacing pulses delivered by ICD 14 may be requiredto have relatively higher pulse energy in order to capture the patient'sheart than pacing pulses delivered using endocardial or epicardial leadsand electrodes. In order to conserve the battery charge of theextra-cardiovascular ICD system 10 for delivering cardioversion anddefibrillation shocks, and promote an intrinsically conducted sinusrhythm over a paced rhythm, ICD 14 operates in a sensing withoutbradycardia pacing mode and only switches to a bradycardia pacing modewhen asystole or a long ventricular pause is detected. Energy may alsobe conserved by not maintaining pacing capacitors in a charged state,ready for pacing pulse delivery, during the sensing without pacing mode.These techniques are disclosed in the context of an extra-cardiovascularICD, such as system 10; however aspects of the techniques disclosedherein may be beneficially implemented in an ICD system that includestransvenous leads carrying endocardial electrodes or non-transvenousleads carrying epicardial electrodes as well. Generally, the techniquesdisclosed herein may be implemented in any ICD system for conservingbattery longevity, promoting intrinsically conducted sinus rhythm andminimizing cardiac pacing while still providing the ability to providecardiac pacing for the patient when asystole or a long ventricular pauseoccurs

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a canelectrode). In examples described herein, housing 15 may be used as anactive can electrode for use in delivering cardioversion/defibrillation(CV/DF) shocks or other high voltage pulses delivered using a highvoltage therapy circuit. In other examples, housing 15 may be availablefor use in delivering unipolar, low voltage cardiac pacing pulses inconjunction with lead-based cathode electrodes and/or for sensingcardiac signals. In other instances, the housing 15 of ICD 14 mayinclude a plurality of electrodes on an outer portion of the housing.The outer portion(s) of the housing 15 functioning as an electrode(s)may be coated with a material, such as titanium nitride. Housing 15 maybe coated with an electrically insulating coating but have an exposedelectrically conductive portion defining the can electrode.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memory, transceivers,sensors, electrical sensing circuitry, therapy delivery circuitry, powersources and other components for sensing cardiac electrical signals,detecting a heart rhythm, and controlling and delivering electricalstimulation pulses to treat an abnormal heart rhythm.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28, 30 and 31. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently. Electrodes 24 and 26 (and in some exampleshousing 15) are referred to herein as defibrillation electrodes becausethey are utilized, individually or collectively, for delivering highvoltage stimulation therapy (e.g., cardioversion or defibrillationshocks and in some cases pacing pulses). Electrodes 24 and 26 may beelongated coil electrodes and generally have a relatively high surfacearea for delivering high voltage electrical stimulation pulses comparedto low voltage pacing and sensing electrodes 28, 30 and 31. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. Electrodes 24 and/or 26 may be used in a pacing electrodevector for delivering extra-cardiovascular pacing pulses, such as backupasystole pacing pulses, bradycardia pacing pulses, anti-tachycardiapacing (ATP) pulses, post-shock pacing pulse, and/or in a sensing vectorused to sense cardiac electrical signals and detect asystole,bradycardia, ventricular tachycardia (VT) and ventricular fibrillation(VF).

Electrodes 28, 30 and 31 are relatively smaller surface area electrodesfor delivering low voltage pacing pulses and for sensing cardiacelectrical signals. Electrodes 28, 30 and 31 are referred to aspace/sense electrodes because they are generally configured for use inlow voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28, 30 and 31 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. A third pace/senseelectrode 31 may be located distal to defibrillation electrode 26.Electrodes 28 and 30 are illustrated as ring electrodes, and electrode31 is illustrated as a hemispherical tip electrode in the example ofFIGS. 1A and 1B. However, electrodes 28, 30 and 31 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong the distal portion 25 of lead 16. Further, electrodes 28, 30 and31 may be of similar type, shape, size and material or may differ fromeach other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of lead 16 may depend on thelocation of ICD 14, the arrangement and position of electrodes carriedby the lead distal portion 25, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, 30 and 31 locatedalong the distal portion 25 of the lead body 18. Lead body 18 may betubular or cylindrical in shape. In other examples, the distal portion25 (or all of) the elongated lead body 18 may have a flat, ribbon orpaddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24 and 26 and pace/sense electrodes 28, 30 and 31. The respectiveconductors electrically couple the electrodes 24, 26, 28, 30 and 31 tocircuitry, such as a therapy delivery circuit and/or a sensing circuit,of ICD 14 via connections in the connector assembly 17, includingassociated electrical feedthroughs crossing housing 15. The electricalconductors transmit therapy from a therapy circuit within ICD 14 to oneor more of defibrillation electrodes 24 and 26 and/or pace/senseelectrodes 28, 30 and 31 and transmit sensed electrical signals from oneor more of defibrillation electrodes 24 and 26 and/or pace/senseelectrodes 28, 30 and 31 to the sensing circuit within ICD 14.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 28, 30, and/or 31. In some examples, housing15 of ICD 14 is used in combination with one or more of electrodes 28,30 and/or 31 in a sensing electrode vector. ICD 14 may even obtaincardiac electrical signals using a sensing vector that includes one orboth defibrillation electrodes 24 and/or 26, e.g., between electrodes 24and 26 or one of electrodes 24 or 26 in combination with one or more ofelectrodes 28, 30, 31, and/or the housing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asasystole, bradycardia, ventricular tachycardia (VT) or ventricularfibrillation (VF). ICD 14 may analyze the heart rate and/or morphologyof the cardiac electrical signals to monitor for tachyarrhythmia inaccordance with any of a number of tachyarrhythmia detection techniques.One example technique for detecting tachyarrhythmia is described in U.S.Pat. No. 7,761,150 (Ghanem, et al.), incorporated by reference herein inits entirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF) according to atachyarrhythmia operating mode. ICD 14 may deliver ATP in response to VTdetection, and in some cases may deliver ATP prior to a CV/DF shock orduring high voltage capacitor charging in an attempt to avert the needfor delivering a CV/DF shock. ATP may be delivered using anextra-cardiovascular pacing electrode vector selected from any ofelectrodes 24, 26, 28, 30, 31 and/or housing 15.

In addition to delivering electrical stimulation therapy for treating VTand VF according to a tachyarrhythmia operating mode, ICD 14 isconfigured to operate in a sensing without pacing mode according to abradycardia operating mode and deliver bradycardia pacing support whenautomatic switching to a pacing mode is enabled. ICD 14 switches fromthe sensing without pacing mode to a pacing mode in response todetecting asystole or a long ventricular pause. Upon switching to thepacing mode, the pacing electrode vector used to deliver cardiac pacingpulses may be the same or different than the sensing electrode vectorused to sense cardiac electrical signals and detect asystole. In oneexample, cardiac electrical signals are sensed between pace/senseelectrodes 28 and 30, and pacing pulses are delivered between pace/senseelectrode 30 used as a cathode electrode and defibrillation electrode 24used as a return anode electrode. In other examples, pacing pulsesdelivered according to a bradycardia operating mode, during a temporarypacing or permanent bradycardia pacing mode, may be delivered betweenpace/sense electrode 28 and either (or both) defibrillation electrode 24or 26 or between defibrillation electrode 24 and defibrillationelectrode 26. These examples are not intended to be limiting, and it isrecognized that other sensing electrode vectors and pacing electrodevectors may be selected according to individual patient need and theavailable electrodes coupled to ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. In otherexamples, lead 16 may include less than three pace/sense electrodes ormore than three pace/sense electrodes and/or a single defibrillationelectrode or more than two electrically isolated or electrically coupleddefibrillation electrodes or electrode segments. The pace/senseelectrodes 28, 30 and/or 31 may be located elsewhere along the length oflead 16. For example, lead 16 may include a single pace/sense electrode30 between defibrillation electrodes 24 and 26 and no pace/senseelectrode distal to defibrillation electrode 26 or proximaldefibrillation electrode 24. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the techniques disclosed herein aredescribed in U.S. Publication No. 2015/0306375 (Marshall, et al.) andU.S. Publication No. 2015/0306410 (Marshall, et al.), both of which areincorporated herein by reference in their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferior from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may alternatively be embodied as a homemonitor or hand held device. External device 40 may be used to programcardiac rhythm detection parameters and therapy control parameters usedby ICD 14. Control parameters used to detect asystole, controlbradycardia pacing mode switching, and control delivery of asystolebackup pacing pulses according to techniques disclosed herein may beprogrammed into ICD 14 using external device 40.

Data stored or acquired by ICD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. As described below, ICD 14 may determine and store asystoleepisode data, including a cardiac electrical signal segment, in responseto detecting asystole. Asystole episode data stored by ICD 14 may betransmitted to external device 40 for review by a clinician.Additionally or alternatively, ICD 14 may transmit a patient orclinician alert or notification to external device 40 in response todetecting a predetermined number of asystole episodes. An asystoledetection notification may be transmitted the first time thepredetermined number of asystole episodes is reached since ICD 14 wasmost recently interrogated by external device 40, or another externaldevice, and the notification may not be transmitted again until afterthe next interrogation session.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement, lead 16 ofsystem 10 is implanted at least partially underneath sternum 22 ofpatient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14toward xiphoid process 20 and at a location near xiphoid process 20bends or turns and extends superiorly within anterior mediastinum 36 ina substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 8.Other implant locations and lead and electrode arrangements that may beused in conjunction with the cardiac pacing techniques described hereinare generally disclosed in the above-incorporated patent applications.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of extra-cardiovascular lead 16 of FIGS. 1A-2C having acurving distal portion 25′ of lead body 18′. Lead body 18′ may be formedhaving a curving, bending, serpentine, or zig-zagging shape along distalportion 25′. In the example shown, defibrillation electrodes 24′ and 26′are carried along curving portions of the lead body 18′. Pace/senseelectrode 30′ is carried in between defibrillation electrodes 24′ and26′. Pace/sense electrode 28′ is carried proximal to the proximaldefibrillation electrode 24′. No electrode is provided distal todefibrillation electrode 26′ in this example.

As shown in FIG. 3, lead body 18′ may be formed having a curving distalportion 25′ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “c.” Defibrillation electrodes 24′and 26′ are each carried by one of the two respective C-shaped portionsof the lead body distal portion 25′, which extend or curve in the samedirection away from a central axis 33 of lead body 18′. In the exampleshown, pace/sense electrode 28′ is proximal to the C-shaped portioncarrying electrode 24′, and pace/sense electrode 30′ is proximal to theC-shaped portion carrying electrode 26′. Pace/sense electrodes 28′ and30′ may, in some instances, be approximately aligned with the centralaxis 33 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24′ and 26′ are laterally offsetfrom electrodes 28′ and 30′. Other examples of extra-cardiovascularleads including one or more defibrillation electrodes and one or morepacing and sensing electrodes carried by curving, serpentine, undulatingor zig-zagging distal portion of the lead body that may be implementedwith the pacing techniques described herein are generally disclosed inU.S. patent application Ser. No. 14/963,303, incorporated herein byreference in its entirety.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 4) may include software, firmware and/or hardwarethat cooperatively monitor one or more cardiac electrical signals,determine when an electrical stimulation therapy is necessary, anddeliver therapies as needed according to programmed therapy deliveryalgorithms and control parameters. The software, firmware and/orhardware are configured to monitor a cardiac electrical signal fordetecting an abnormal heart rhythm and configured to select and deliveran appropriate electrical stimulation therapy for treating the abnormalrhythm. ICD 14 is coupled to an extra-cardiovascular lead, such as lead16 carrying extra-cardiovascular electrodes 24, 26, 28, 30 and 31, fordelivering electrical stimulation pulses to the patient's heart and forsensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, cardiac electrical signal sensing circuit 86, telemetrycircuit 88, patient notification circuit 90 and sensor 92. A powersource 98 provides power to the circuitry of ICD 14, including each ofthe components 80, 82, 84, 86, 88, 90 and 92 as needed. Power source 98may include one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 98 and each of the other components 80, 82, 84, 86, 88, 90and 92 are to be understood from the general block diagram of FIG. 4,but are not shown for the sake of clarity. For example, power source 98may be coupled to a low voltage charging circuit and to a high voltagecharging circuit included in therapy delivery circuit 84 for charginglow voltage and high voltage capacitors, respectively, included intherapy delivery circuit 84 for producing respective low voltage pacingpulses, such as asystole backup pacing pulses, bradycardia pacingpulses, post-shock pacing or ATP pulses, or for producing high voltagepulses, such as CV/DF shock pulses, and in some cases high voltagepacing pulses. In some examples, high voltage capacitors are charged andutilized for delivering pacing pulses for providing asystole backuppacing instead of low voltage capacitors. Power source 98 additionallypowers one or more processors and/or other control circuits included incontrol circuit 80 as well as other components of sensing circuit 86,telemetry circuit 88, memory 82, and therapy delivery circuit 84 asneeded.

The functional blocks shown in FIG. 4 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combination of any of the aforementioned components that provide thedescribed functionality. The particular form of software, hardwareand/or firmware employed to implement the functionality disclosed hereinwill be determined primarily by the particular system architectureemployed in the device and by the particular detection and therapydelivery methodologies employed by the ICD 14. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern ICD system, given the disclosure herein, iswithin the abilities of one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. For example, memory 82 may include read-onlymemory (ROM) in which stored programs controlling the operation of thecontrol circuit 80 reside. Memory 82 may further include random accessmemory (RAM) or flash memory configured as a number of recirculatingbuffers capable of holding a series of measured intervals for analysisby the control circuit 80 for predicting or diagnosing an arrhythmia.Furthermore, memory 82 may include non-transitory computer readablemedia storing instructions that, when executed by one or more processingcircuits, cause control circuit 80 or other ICD components to performvarious functions attributed to ICD 14 or those ICD components. Thenon-transitory computer-readable media storing the instructions mayinclude any of the media listed above.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as components isintended to highlight different functional aspects and does notnecessarily imply that such components must be realized by separatehardware, firmware or software components. Rather, functionalityassociated with one or more components may be performed by separatehardware, firmware or software components, or integrated within commonhardware, firmware or software components. For example, therapy deliveryoperations may be performed by therapy delivery circuit 84 under thecontrol of control circuit 80 and may include operations implemented ina processor executing instructions stored in memory 82 and controlsignals such as timing signals, pacing pulse amplitude signals, andpacing electrode vector selection signals sent from control circuit 80to therapy delivery circuit 84.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, and 30 (and 31 ifpresent) carried by lead 16 (e.g., as shown in FIG. 1A) and the housing15, which may function as a common or ground electrode for deliveringelectrical pulses or as an active can electrode for delivering CV/DFshock pulses.

Cardiac electrical signal sensing circuit 86, also referred to herein as“electrical signal sensing circuit” or simply “sensing circuit,” may beselectively coupled to electrodes 28, 30 (and 31 if present) and/orhousing 15 in order to monitor electrical activity of the patient'sheart. Electrical signal sensing circuit 86 may additionally beselectively coupled to defibrillation electrodes 24 and/or 26 for use ina sensing electrode vector. Sensing circuit 86 is enabled to selectivelymonitor one or more sensing vectors at a time selected from theavailable electrodes 24, 26, 28, 30, 31 and housing 15. For example,sensing circuit 86 may include switching circuitry for selecting whichof electrodes 24, 26, 28, 30 and 31 (shown in FIG. 1) and housing 15 arecoupled to sense amplifiers or other cardiac event detection circuitryincluded in sensing circuit 86. Switching circuitry may include a switcharray, switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple components of sensing circuit 86 toselected electrodes. In some instances, control circuit 80 may controlthe switching circuitry to selectively couple sensing circuit 86 to oneor more sense electrode vectors. The cardiac event detection circuitrywithin sensing circuit 86 may include one or more sense amplifiers,filters, rectifiers, voltage amplitude threshold detectors, comparators,analog-to-digital converters (ADCs), or other analog or digitalcomponents.

In some examples, sensing circuit 86 includes multiple sensing channelsfor acquiring cardiac electrical signals from multiple sensing vectorsselected from electrodes 24, 26, 28, 30 (and 31 if present) and housing15. Each sensing channel may be configured to amplify, filter andrectify the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor sensing cardiac events, e.g., R-waves. For example, each sensingchannel may include a pre-filter and amplifier for filtering andamplifying a signal received from a selected pair of electrodes. Theresulting raw cardiac electrical signal may be passed from thepre-filter and amplifier to cardiac event detection circuitry forsensing cardiac events from the received cardiac electrical signal.Cardiac event detection circuitry may include a rectifier, post-filterand amplifier, a sense amplifier, comparator, and/or analog-to-digitalconverter for detecting a cardiac event when the cardiac electricalsignal crosses a sensing threshold. The sensing threshold may be set bycontrol circuit 80, based on a value stored in memory 82 which may beprogrammed by a user, and passed from control circuit 80 to sensingcircuit 86 via a data bus. Sensing circuit 86 may include anauto-adjusting sense amplifier or detector that compares the cardiacsignal to a sensing threshold that decays or decreases from a startingvalue to a minimum sensing floor in some examples.

Upon detecting a cardiac event, sensing circuit 86 may produce a sensedevent signal, such as an R-wave sensed event signal 91, that is passedto control circuit 80. The sensed event signals 91 are used by controlcircuit 80 for detecting cardiac rhythms and determining a need fortherapy. Electrical signal sensing circuit 86 may also pass a digitizedelectrocardiogram (ECG) signal to control circuit 80 for morphologyanalysis performed for detecting and discriminating heart rhythms.

Additionally or alternatively, electrical signals received by sensingcircuit 86 from a selected sensing electrode vector may be passedthrough a bandpass filter and amplifier, provided to a multiplexer andthereafter converted to multi-bit digital signals by ananalog-to-digital converter, all included in sensing circuit 86, forstorage in random access memory or flash memory included in memory 82under control of a direct memory access circuit via a data/address bus.The digitized cardiac electrical signal may be analyzed by controlcircuit 80 for identifying cardiac events such as R-waves and fordetecting and discriminating abnormal rhythms, such as asystole, VT andVF.

Control circuit 80 may be a microprocessor-based controller that employsdigital signal analysis techniques to characterize the digitized signalsstored in memory 82 to recognize and classify the patient's heart rhythmemploying any of numerous signal processing methodologies for analyzingcardiac signals and cardiac event waveforms, e.g., R-waves. Onetachyarrhythmia detection system is described in U.S. Pat. No. 5,545,186(Olson et al.), incorporated herein by reference in its entirety.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices, such as one or more high voltage capacitorsand/or one or more low voltage capacitors, and switching circuitry thatcontrols when the capacitor(s) are discharged across a selected pacingelectrode vector or CV/DF shock vector. Charging of capacitors to aprogrammed pulse amplitude and discharging of the capacitors for aprogrammed pulse width may be performed by therapy delivery circuit 84according to control signals received from control circuit 80. Controlcircuit 80 may include various timers or counters that control whenpacing pulses are delivered.

For example, control circuit 80 may include pacer timing and controlcircuitry having programmable digital counters set by the microprocessorof the control circuit 80 for controlling the basic time intervalsassociated with various pacing modes or anti-tachycardia pacingsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

During a pacing mode, escape interval counters within the pacer timingand control circuitry are reset upon sensing of R-waves as indicated bysignals 91 from sensing circuit 86. When the escape interval times out,a pacing pulse is generated by a pulse output circuit of therapydelivery circuit 84. The pace output circuit is coupled to the desiredelectrodes via switch matrix for discharging one or more capacitorsacross the pacing load. The escape interval counters are reset upongeneration of pacing pulses, and thereby control the basic timing ofcardiac pacing functions, including asystole backup pacing during atemporary pacing mode, permanent bradycardia pacing modes, and ATP orpost-shock pacing controlled according to a tachyarrhythmia operatingmode. The durations of the escape intervals are determined by controlcircuit 80 via a data/address bus. The value of the count present in theescape interval counter when reset by a sensed R-wave can be used tomeasure an RR interval as the time interval between two consecutivelysensed R-waves. RR intervals are determined for detecting the occurrenceof a variety of arrhythmias and for storing RR interval data associatedwith a detected arrhythmia episode, including asystole episodes.

In some examples, control circuit 80 may detect asystole when operatingin a sensing without pacing mode when a counter or timer started uponreceiving an R-wave sensed event signal 91 reaches or times out apredetermined asystole detection time interval, such as an interval of 2seconds, an interval of 3 seconds or an interval of 4 seconds or more.The asystole detection time interval used for detecting asystole bycontrol circuit 80 may be programmable and stored in memory 82 and maybe up to six seconds long in some examples. Other techniques may beimplemented for detecting asystole. For example, aspects disclosed inU.S. Pat. No. 8,831,713 (Stadler, et al.), incorporated herein byreference in its entirety, may be employed for detecting asystole andavoiding false asystole detection. In response to detecting asystole,control circuit 80 switches to a pacing mode if asystole backup pacingis enabled and controls therapy delivery circuit 84 to deliver pacingpulses according to programmed pacing escape intervals in the absence ofan R-wave sensed event signal.

Therapy delivery circuit 84 may be controlled by control circuit 80 todeliver pacing pulses via a pacing electrode vector selected from theextra-cardiovascular electrodes 24, 26, 28, 30 (and 31 if present)and/or housing 15 using a high voltage output circuit capable of alsodelivering cardioversion and defibrillation shocks. The pacing pulse maybe delivered as a single pulse, which may be a balanced biphasic pulsein some instances, by enabling the high voltage output circuit todeliver a pacing pulse that is much lower in energy than acardioversion/defibrillation shock. Enabling the high-voltage pacingoutput circuit of therapy delivery circuit 84 to deliver a pacing pulseduring a temporary pacing mode enabled in response to detecting asystolemay include setting a variable shunt resistance included in the highvoltage output circuit to maintain a minimum electrical current toswitches included in the high voltage pacing output circuit, e.g., in anH-bridge, used to couple previously charged high voltage capacitor(s) tothe pacing electrode vector for delivery of the pacing pulse. Thevariable resistance may be set to match a pacing load so that electricalcurrent through the switches may be maintained at or above a minimumelectrical current required to maintain a stable closed state of desiredswitches during the pacing pulse. Techniques for delivering the pacingpulse using a high voltage output circuit of therapy delivery circuit 84are generally disclosed in provisionally-filed U.S. Pat. Application No.62/262,499 (Anderson, et al.), incorporated herein by reference in itsentirety.

In some examples, therapy delivery circuit 84 may include a low voltageoutput circuit for delivering pacing pulses during a temporary orpermanent bradycardia pacing mode, as well as for post-shock pacing, ATPor other pacing pulses. The low voltage output circuit may include a lowvoltage capacitor array for delivering pacing pulses. Control circuit 80may select a combination of capacitors from the capacitor array so thatan RC time constant based on the pacing electrode vector impedance andthe selected capacitors results in a pacing pulse width that is longenough to capture when the pacing pulse amplitude is less than a painthreshold of the patient. By selecting a capacitor combination that hasan RC time constant that results in a relatively slow decay rate of thepulse, the pacing pulse amplitude at truncation of the pacing pulseafter a relatively long pulse width, e.g., 1.5 ms or longer, is stillgreater than the pacing capture threshold of the heart.Extra-cardiovascular pacing may be delivered at a pacing pulse amplitudebelow a pain threshold of the patient with a pulse width long enough todeliver adequate energy to successfully pace the heart. ICD 14 may beconfigured to deliver pacing pulses using extra-cardiovascularelectrodes according to the techniques generally disclosed in U.S.patent application Ser. No. 14/957,651 (Christie, et al.), incorporatedherein by reference in its entirety.

In other examples, therapy delivery circuit 84 may include a low voltageoutput circuit including at least two holding capacitors or at least twoholding capacitor combinations that are charged by the low voltagecharging circuit to a pacing pulse amplitude under the control ofcontrol circuit 80 and discharged sequentially to deliver at least twoindividual pulses that are fused in time to produce a composite pacingpulse having a total pulse width that is longer than the pulse widthcapture threshold of the heart. A series of fused low voltage electricalpulses may be delivered using extra-cardiovascular electrodes 24, 26,28, and/or 30 (and 31 if available) and/or housing 15 to produce acomposite cardiac pacing pulse having a total pulse width defined by thefused low voltage pulses. Extra-cardiovascular pacing pulses may bedelivered by therapy delivery circuit 84 at a pacing pulse amplitudebelow a pain threshold of the patient and a total pulse width defined bythe composite pacing pulse that is long enough to deliver adequateenergy to successfully capture and pace the heart. Techniques fordelivering fused pacing pulses using extra-cardiovascular electrodesthat may be coupled to ICD 14 are generally disclosed inprovisionally-filed U.S. Pat. Application No. 62/262,412 (Anderson, etal.), incorporated herein by reference in its entirety. Therapy deliverycircuit 84 may include other types of pacing output circuitry than theexamples given here for delivering pacing pulses using either lowvoltage capacitors, high voltage capacitors or a combination of both.The methods for detecting asystole and controlling an asystole backuppacing mode and a sensing without pacing mode as described herein may beimplemented in conjunction with a variety of pacing output circuits.

In other examples, when ICD 14 is coupled to transvenous leads carryingendocardial electrodes, pacing pulses may be delivered using alow-voltage pacing output circuit configured to deliver pacing pulsesthat are typically less than 2.0 ms in pulse width and less than 8.5 Vin pulse amplitude.

In response to the detection of asystole during a sensing without pacingmode, a backup asystole pacing therapy may be delivered after switchingto a temporary pacing mode by loading pacing control parameters, whichmay be stored within memory 82, from the microprocessor included incontrol circuit 80 into the pacer timing and control circuit accordingto the pacing pulse amplitude, pacing pulse width, and one or morepacing escape intervals for controlling the timing of the pacing pulsesas described in conjunction with FIGS. 7 and 8, for example. In responseto detecting a threshold number of asystole episodes during the sensingwithout pacing mode, bradycardia pacing pulses may be delivered afterswitching to a permanent bradycardia pacing mode. During the temporarypacing mode, or during the permanent bradycardia pacing mode, the pacertiming and control circuit may set an escape interval counter when anR-wave sensed event signal 91 is received by control circuit 80. If theescape interval expires without receiving another R-wave sensed eventsignal, therapy delivery circuit 84 is enabled to discharge one or morecapacitors across the selected pacing electrode vector.

In response to detection of a tachyarrhythmia, ATP pulses may bedelivered by therapy delivery circuit 84 under the control of controlcircuit 80 according to a programmed ATP therapy sequence stored inmemory 82. In the event that higher voltage cardioversion ordefibrillation pulses are required, the control circuit microprocessoractivates cardioversion and defibrillation control circuitry included incontrol circuit 80 to initiate charging of the high voltage capacitorsvia a charging circuit, both included in therapy delivery circuit 84,under the control of a high voltage charging control line. The voltageon the high voltage capacitors is monitored via a voltage capacitorline, which is passed to control circuit 80. When the voltage reaches apredetermined value set by the microprocessor of control circuit 80, alogic signal is generated on a capacitor full line passed to therapydelivery circuit 84, terminating charging. The defibrillation orcardioversion pulse is delivered to the heart under the control of thepacer timing and control circuitry by an output circuit of therapydelivery circuit 84 via a control bus. The output circuit determines theelectrodes used for delivering the cardioversion or defibrillation pulseand the pulse wave shape. Therapy delivery and control circuitrygenerally disclosed in any of the above-incorporated patents may beimplemented in ICD 14.

In some examples, ICD 14 includes a sensor 92 capable of producing asignal correlated to patient activity and/or patient posture. A signalfrom sensor 92 may be used to determine a sensor-indicated pacing ratethat is used to control the rate of pacing pulses delivered by therapydelivery circuit 84 when rate responsive bradycardia pacing is enabledduring a temporary or permanent pacing mode. Sensor 92 may be anaccelerometer or other motion sensor. For instance, sensor 92 may be aone-dimensional, two-dimensional, or three-dimensional piezoelectricsensor or micro electro-mechanical systems (MEMS) sensor that generatesa motion signal corresponding to acceleration or motion of the patientin one, two or three axes, respectively. Examples of a sensor and methodfor determining patient activity and posture are generally disclosed inU.S. Pat. No. 5,593,431 (Sheldon), incorporated herein by reference inits entirety. A piezoelectric accelerometer for detecting patient motionis disclosed, for example, in U.S. Pat. No. 4,485,813 (Anderson, et al.)and U.S. Pat. No. 5,052,388 (Sivula, et al.), both of which areincorporated herein by reference in their entirety. The use of a patientactivity signal for providing rate-responsive pacing is generallydisclosed in U.S. Pat. No. 7,031,772 (Condie, et al.), incorporatedherein by reference in its entirety.

Control circuit 80 may be configured to determine one or more restconditions from a signal received from sensor 92. One rest condition maybe an activity state of the patient. Another rest condition may be abody posture of the patient. Control circuit 80 may be configured todetermine whether the patient is in a rest state or a non-rest statebased on the activity state of the patient, the body posture of thepatient, and/or a time of day in some examples. Control circuit 80 mayinclude a 24-hour timer or clock initialized to a time of day fordetermining the time of day.

In response to the patient activity determined from a signal from sensor92 being below an activity threshold level, the patient body posturebeing in a non-upright posture (e.g., a lying or reclined position asopposed to a sitting, standing or walking position), and/or the time ofday being night time, a resting state of the patient may be detected bycontrol circuit 80. If the patient activity is greater than a thresholdlevel, the patient is in an upright posture, and/or the time of day isdaytime, control circuit 80 may detect a non-resting state of thepatient. As described below in conjunction with FIGS. 9 and 10, thedetermined resting or non-resting state of the patient may be used tocontrol asystole detection parameters, automatic switching to atemporary pacing mode for delivering asystole backup pacing pulses,and/or pacing control parameters used to control delivery of asystolebackup pacing pulses after switching to a temporary pacing mode.

Control parameters utilized by control circuit 80 for detecting cardiacrhythms and controlling therapy delivery may be programmed into memory82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

Control circuit 80 may terminate a temporary pacing mode that has beenstarted upon asystole detection in response to a user commandtransmitted to ICD 14 from external device 40. Telemetry circuit 88receives the command and passes command signal data to control circuit80 for terminating the temporary pacing mode. Control circuit 80 mayinclude a reed switch or Hall-effect sensor that may be activated by auser externally applying a magnet over ICD 14. Control circuit 80 mayterminate the temporary pacing mode that was started upon asystoledetection when a magnet being applied to the ICD 14 is detected based onthe status of a Hall-effect sensor or reed switch or another magnetdetection element included in control circuit 80. When the temporarypacing mode is terminated, control circuit 80 restores the sensingwithout pacing mode.

In some examples, the ICD 14 may be equipped with a patient notificationcircuit 90 that generates a signal perceivable by the patient to notifythe patient of an important clinical event or ICD status that maywarrant medical attention. For instance, control circuit 80 may controlpatient notification circuit 90 to issue an audible alert in response todetecting asystole. Any patient notification method used in implantablemedical devices may be implemented in notification circuit 90 such asgenerating a perceivable twitch stimulation or an audible sound. Apatient notification system may include an audio transducer that emitsaudible sounds including voiced statements or musical tones stored inmemory 82 corresponding to a notification triggering event, such as anasystole episode. A patient notification system is generally describedin U.S. Pat. No. 6,067,473 (Greeninger et al.), incorporated herein byreference in its entirety.

FIG. 5A and FIG. 5B are diagrams of a cardiac electrical signal 110 anda corresponding cardiac event timeline 120 depicting R-wave sensed eventsignals (R) 122 and pacing pulses (P) 128 generated by ICD 14. As shownin FIG. 5A, cardiac electrical signal 110 includes R-waves 112 that aresensed by the sensing circuit 86 of ICD 14. During a sensing withoutpacing mode 102, sensing circuit 86 produces an R-wave sensed eventsignal 122 that is passed to control circuit 80 each time a respectiveR-wave 112 is sensed.

In response to each R-wave sensed event signal 122, control circuit 80may start a probable asystole time interval 124 and an asystoledetection time interval 125. One or both of the probable asystole timeinterval 124 and the asystole detection time interval 125 may beprogrammable and may be set to the same or different time intervals.Both are reset upon an R-wave sensed event signal 122. If the probableasystole time interval 124 expires at time 126, before being reset dueto an R-wave sensed event signal 122, the control circuit 80 enablesasystole episode data storage at time 126. Asystole episode data storagemay include storing a segment 127 of cardiac signal 122. The segment 127of cardiac electrical signal 122 that is stored in memory 82 may extendfrom time 126 or a predetermined interval prior to the expiration of theprobable asystole timer 124 to include cardiac events leading up to theasystole and may extend a predetermined time interval after theexpiration of the probable asystole timer 124, after an asystoledetection is made at time 142, or after switching to a temporary pacingmode 104 in response to detecting the asystole.

If the asystole detection time interval 125 expires without an R-wavesensed event signal 122, asystole is detected at time 142. In responseto detecting asystole, control circuit 80 may switch from the sensingwithout pacing mode 102 to a temporary pacing mode 104. A pacing escapeinterval 132 is started upon switching to the temporary pacing mode 104,and pacing pulse 128 is delivered if the escape interval 132 expireswithout an R-wave sensed event signal occurring during the escapeinterval 132. The pacing escape interval 132 is set to a programmedasystole backup pacing rate interval, which may be the longest availablepacing rate interval corresponding to a minimum available pacing rate.In other examples, the asystole backup rate interval used to set pacingescape interval 132 is set to be equal a programmed bradycardia pacinglower rate interval.

In some instances, instead of starting the first pacing escape interval132 upon switching to the temporary pacing mode 104 at time 142, apacing pulse may be immediately delivered upon switching to thetemporary pacing mode 104, and the first pacing escape interval may bestarted after delivering the first pacing pulse. Charging of capacitorsincluded in therapy delivery circuit 84 may begin after the probableasystole timer 124 expires so that a pacing pulse may be deliveredimmediately upon switching to the temporary pacing mode if asystole isdetected. The pacing pulses delivered during the temporary pacing mode104 may capture the ventricles of the patient's heart 8 and correspondto a VVI, VVIR, VVT or VVTR pacing mode. In some examples, a terminationtime interval 140 set to a predetermined maximum temporary pacing timeperiod may be started upon switching to the temporary pacing mode 104.

In the VVI mode or VVIR mode (VVI plus rate responsive pacing), a pacingpulse is inhibited when an intrinsic R-wave is sensed during a pacingescape interval. In the VVT or VVTR mode, a sensed event triggers animmediate pacing pulse, which starts a pacing escape interval. Thepacing pulse delivered immediately occurs in the physiologicalrefractory period of the myocardium after the sensed event e.g., within100 ms or within no more than 200 ms of the sensed event, and subsequentpacing pulses may be delivered at a rate that is faster than theintrinsic rate. This VVT or VVTR mode may be used in situations whenP-wave oversensing or other false sensing of events as R-waves may bepresent. Oversensing of cardiac or non-cardiac noise may result in apacing pulse being inhibited during a VVI(R) pacing mode. By using aVVT(R) pacing mode, inhibition of pacing pulses due to oversensed eventsthat are not true R-waves is prevented.

To illustrate, when operating in a VVI mode, if control circuit 80receives an R-wave sensed event signal from sensing circuit 86 during apacing escape interval, e.g., the first pacing escape interval 132 shownin FIG. 5A, after switching to the temporary pacing mode 104, the firstpacing pulse 128 shown in FIG. 5A would be inhibited (not delivered).The pacing escape interval 132 would be restarted without delivering thescheduled pacing pulse. When operating in a VVT mode, if control circuit80 receives an R-wave sensed event signal during the first pacing escapeinterval 132 shown in FIG. 5A (or any other pacing escape interval)after switching to the temporary pacing mode 104, the scheduled pacingpulse 128 is triggered to be delivered immediately, before the pacingescape interval 132 expires so that the pacing pulse is delivered withina physiological refractory period of the sensed event. Delivery of thetriggered pacing pulse causes the pacing escape interval 132 to berestarted.

Upon delivering a pacing pulse 128, the pacer timing and controlcircuitry included in control circuit 80 starts a next pacing escapeinterval 132 as shown in FIG. 5B. If an intrinsic R-wave is sensedduring the next pacing escape interval 132, the pacing escape interval132 is reset and the scheduled pacing pulse is inhibited when the pacingmode is VVI(R). If pacing escape interval 132 expires without sensing anintrinsic R-wave, however, a pacing pulse 128 is delivered by therapydelivery circuit 84. In other examples, an R-wave sensed event signalmay trigger delivery of a pacing pulse when the temporary pacing mode isprogrammed to be a VVT(R) pacing mode. The pacing pulses 128 may all bedelivered at the same pacing escape interval 132 in some cases. In othercases, as described below in conjunction with FIGS. 7 and 8, the pacingescape interval may be set to different values during the temporarypacing mode 104 such that pacing pulses may be delivered at differentrates corresponding to the respective values of the pacing escapeinterval.

During the temporary pacing mode 104, control circuit 80 may continuewriting a segment 127 of the cardiac electrical signal 110 to memory 82to record the asystole episode and the pacing response. In otherexamples, the recorded segment 127 of the cardiac electrical signal 110may terminate upon switching to the temporary pacing mode 104 or after apredetermined number of pacing pulses or other predetermined time limit.Control circuit 80 may monitor for one or more pacing mode terminationconditions after switching to the temporary pacing mode 104. In oneexample, expiration of the termination time interval 140 is a temporarypacing mode termination condition. As such, upon expiration of thetermination time interval 140 at 146, the control circuit 80automatically switches from the temporary pacing mode 104 back to thesensing without pacing mode 102 as shown in FIG. 5B. Alternatively,control circuit 80 may include a counter for counting a predeterminedmaximum number of pacing pulses that can be delivered during thetemporary pacing mode before switching back to the sensing withoutpacing mode. Other temporary pacing mode termination conditions arediscussed below in conjunction with FIG. 6 and may include detecting atachyarrhythmia episode, detecting a predetermined number of intrinsicR-waves, or upon a user request.

During the sensing without pacing mode 102, one or more RR intervals 130may occur that are longer than the pacing escape interval 132 set to theasystole backup pacing interval during the temporary pacing mode 104.For example, pacing escape interval 132 may be set to a relatively longinterval, such as 1.5 to 2 seconds, to provide asystole backup pacing ata minimal rate, e.g., 30 to 40 paces per minute. RR interval 130 may begreater than the pacing escape interval 132 but is less than theasystole detection time interval 124, which may be up to two seconds, upto three seconds, up to four seconds, or even up to 6 seconds or longerin some examples. Even when an RR interval 130 is longer than aprogrammed asystole backup rate interval, a bradycardia lower rateinterval or other pacing interval used to set pacing escape intervaltimers during temporary pacing mode 104 or another bradycardia pacingmode, no pacing pulses are scheduled or delivered during the sensingwithout pacing mode. Bradycardia pacing pulses are not delivered duringthe sensing without pacing mode, however it is recognized that, if VT orVF is detected, ATP pulses or post-shock pacing pulses may be deliveredduring the sensing without pacing mode in accordance with programmedtachyarrhythmia therapy control parameters.

The first pacing pulse 128 shown in FIG. 5A is delivered during thetemporary pacing mode 104, upon expiration of the first pacing escapeinterval 132 after switching to the temporary pacing mode, in this casea VVI temporary pacing mode. Pacing pulses may or may not be deliveredafter switching to the temporary pacing mode 104 depending on whether ornot intrinsic R-waves are sensed during the first pacing escape interval132 set upon switching to the temporary pacing mode (FIG. 5A) and inresponse to intrinsic R-waves sensed thereafter. In some instances, noneor only the single, first pacing pulse 128 shown in FIG. 5A is deliveredduring the temporary pacing mode 104 due to intrinsic ventricularactivity sensed prior to the expiration of all other pacing escapeintervals set during the temporary pacing mode 104.

The pacing escape interval 132 may be set to an asystole backup rateinterval, e.g., 2 seconds for an asystole backup pacing rate of 30pulses per minute or 1.5 seconds for an asystole backup pacing rate of40 pulses per minute. However, other asystole backup rate intervals maybe used. In other examples, the pacing escape interval 132 may be set toa sensor-indicated pacing rate according to a rate responsive pacingmode, e.g., VVIR. Sensor 92 may provide a signal to control circuit 80.A sensor-indicated pacing rate is determined by control circuit 80 fromthe sensor signal and a rate transfer function stored in memory 82.Activity-based pacing rates may be determined as generally disclosed inthe above-incorporated U.S. Pat. No. 7,031,772 (Condie, et al.). Othermethods for using an accelerometer for monitoring patient activity forcontrolling pacing rate are generally disclosed in U.S. Pat. No.5,562,711 (Yerich, et al.) and U.S. Pat. No. 6,449,508 (Sheldon, etal.), both of which are incorporated herein by reference.

In some examples, the pacing escape interval 132 may be controlledaccording to an asystole backup rate interval and a hysteresis interval.When the pacing escape interval 132 set to the asystole backup rateinterval is restarted due to an R-wave sensed event signal 122 prior toexpiration of the pacing escape interval 132, the pacing escape interval132 is set to a hysteresis interval that is longer than the asystolebackup rate interval to promote intrinsically conducted heart beats tooccur without pacing. As long as the intrinsic heart rate is faster thanthe hysteresis rate, pacing is inhibited. If a hysteresis intervalexpires without a sensed intrinsic R-wave, a pacing pulse is deliveredby the therapy delivery circuit 84 and the pacing escape interval 132 isset to the asystole backup rate interval. The first occurrence of anexpired hysteresis time interval and delivered pacing pulse at thehysteresis rate may suspend hysteresis operations; control circuit 80may reestablish the asystole backup rate interval as the pacing escapeinterval 132. Pacing pulses are delivered at a rate corresponding to theasystole backup rate interval until the temporary pacing mode isterminated or another R-wave sensed event signal is received by thecontrol circuit 82, causing the pacing escape interval to be reset tothe hysteresis interval. Examples of the use of a hysteresis intervalare described below in conjunction with FIGS. 7 and 8.

In still other examples, the pacing escape interval 132 may be set basedon RR intervals determined during the sensing without pacing mode 102,prior to the most recent R-wave sensed event before asystole detection.For example, intervals between consecutive R-waves 122 shown in FIG. 5Amay be determined and used for setting the pacing escape interval 132.For example, pacing escape interval 132 may be set as an average of theRR intervals preceding asystole detection or a predetermined interval orpercentage longer than the average of RR intervals preceding asystoledetection.

FIG. 6 is a flow chart 200 of a method for controlling cardiac pacing byan ICD, such as the extra-cardiovascular ICD 14 of FIGS. 1A and 1B, toprovide asystole backup pacing in response to detecting asystole. Atblock 202, the ICD is operating in the sensing without pacing mode. Thesensing without pacing mode may be a programmable pacing mode or adefault pacing mode of the ICD. The sensing without pacing mode maycorrespond to an OVO or ODO pacing mode in which no pacing pulses aredelivered even when an RR interval is longer than a programmed asystolebackup rate interval or a programmed bradycardia lower rate interval.The asystole backup rate interval used during a temporary pacing modeand the lower rate interval used during a permanent bradycardia pacingmode prevent the heart rate from going below a corresponding minimumrate, e.g., 30 beats per minute or 40 beats per minute, by deliveringpacing pulses at the respective asystole backup rate or lower rate inthe absence of sensed R-waves. During the sensing without pacing mode,pacing escape intervals are not set for scheduling a pacing pulse and apacing pulse is not delivered even if the heart rate falls below theasystole backup rate or the lower rate programmed for use during atemporary or permanent pacing mode, respectively.

Upon enabling the sensing without pacing mode at block 202, an asystoletimer is started at block 203. In the example described in FIG. 6, asingle asystole timer is started that is used for both detectingasystole and for starting the recording of asystole episode data.However, as shown in FIG. 5 and described in further detail in FIG. 9,both a probable asystole time interval 124 and an asystole detectiontime interval 125 may be started. The probable asystole time intervalmay expire earlier than the asystole detection time interval and be usedto control when recording of cardiac electrical signal episode databegins in anticipation of an asystole detection.

If an R-wave is sensed by the sensing circuit 86 at block 204, theasystole timer started at block 203 is restarted during the sensingwithout pacing mode as described in conjunction with FIG. 5. If theasystole timer has not expired, “No” branch of block 208, and anotherR-wave is sensed, “Yes” branch of block 204, the asystole timer isrestarted at block 206. This process continues as long as the asystoletimer does not expire. The ICD remains in the sensing without pacingmode.

If an R-wave is has not been sensed at block 204 before the asystoletimer expires, “Yes” branch of block 208, the control circuit 80 detectsasystole at block 210. Other asystole detection criteria may be requiredto be met in order to detect asystole at block 210. For example, if theasystole timer expires at block 208, control circuit 80 may verify atblock 209 that a tachyarrhythmia episode is not being detected and thatan anti-tachyarrhythmia therapy, which may include ATP or a shocktherapy followed by post-shock pacing, is not being delivered beforedetecting the asystole at block 210. Asystole may be detected at block210 during the sensing without pacing mode in response to an asystoledetection threshold time interval expiring subsequent to a sensed R-wavewhen a tachyarrhythmia episode is not being detected and when atachyarrhythmia therapy, e.g., ATP or a shock therapy and post-shockpacing, is not being delivered.

If a tachyarrhythmia detection is being made or an anti-tachyarrhythmiatherapy is being delivered, “yes” branch of block 209, control circuit80 may withhold switching to the temporary pacing mode by returning toblock 203 to restart the asystole timer. It is to be understood that thecheck to determine if a tachyarrhythmia episode is being detected or ananti-tachyarrhythmia therapy is being delivered, as indicated at block209, is not necessarily limited to a certain time point in the processof detecting asystole during the sensing without pacing mode. Forexample, a tachyarrhythmia detection may be made after detectingasystole at block 210 and an anti-tachyarrhythmia therapy may bescheduled or in process prior to switching to the sensing without pacingmode. In this case, control circuit 80 may withhold switching to thetemporary pacing mode and controls the therapy delivery circuit 84according to anti-tachyarrhythmia therapy control parameters, e.g., ATPtherapy control parameters and/or shock therapy and post-shock pacingtherapy control parameters.

It is recognized that ICD 14 may be configured to deliver post-shockpacing to aid the heart in recovering from acardioversion/defibrillation shock, which can sometimes be followed byasystole. A tachyarrhythmia episode detection may still be in progressafter shock delivery until normal sinus rhythm is detected. As such, ashock followed by asystole and post-shock pacing is considered to bewithin a tachyarrhythmia episode and under the control of atachyarrhythmia mode of operation. In this situation, it is the shockpulse that is the leading event of the asystolic period, not an R-wavesensed outside of a tachyarrhythmia episode. Therapy delivery circuit 84may deliver anti-tachyarrhythmia therapy utilizing low voltage and/orhigh voltage capacitors of therapy delivery circuit 84 such thatcapacitors and other output circuitry of therapy delivery circuit 84 arenot controlled to operate in a temporary pacing mode of the bradycardiaoperating modes simultaneously with anti-tachyarrhythmia therapydelivery. The control circuit 80 may remain in the sensing withoutpacing mode of the bradycardia operating modes when a tachyarrhythmiaepisode or therapy is in progress, including during post-shock pacing.

In some examples, asystole episode data is stored in memory 82 at block211. Asystole episode data may include a segment of the cardiacelectrical signal including at least a portion of the asystole episodealong with a time and date stamp. The segment of the cardiac electricalsignal may include at least a portion of the asystole detection timeinterval preceding the asystole detection, a portion of the cardiacelectrical signal prior to the sensed R-wave that started the asystoledetection time interval, and/or a portion of the cardiac electricalsignal after switching to a temporary asystole backup pacing mode asdescribed below. Other data that may be stored may include an averageheart rate prior to the asystole, the number of asystole detections overa predetermined period of time, e.g., one day, one week, one month,etc., patient activity data, and/or patient posture data. If thetemporary pacing mode is a rate responsive mode, e.g., VVIR or VVTR, thepatient activity data may be used in establishing a pacing escapeinterval meeting the patient's metabolic demand. Asystole episode datastorage is not triggered in response to post-shock pacing associatedwith tachyarrhythmia therapy. Asystole episode data storage at block 211is triggered by detection of asystole during the sensing without pacingmode, which withholds asystole detection during a post-shock period toallow the tachyarrhythmia operating mode to control therapy delivery.

In addition to or alternatively to storing asystole episode data atblock 211, a patient and/or clinician notification may be generated atblock 211 upon detecting asystole at block 210. A notification to thepatient or clinician may be transmitted via telemetry circuit 88 to anexternal device. In some examples, a patient notification is generatedby patient notification circuit 90, e.g., as an audible tone, vibrationor muscle twitch stimulation. A patient and/or clinician notificationmay be generated only in response to the first asystole detection madesince a most recent ICD interrogation and telemetry session with theexternal device 40. No further notifications are generated in responseto subsequent asystole detections until another ICD interrogationsession occurs. In other examples, a notification is generated only whenthe number of asystole detections reaches a threshold number or othernotification criteria are met, such as a threshold number of pacingpulses delivered during one or more ventricular pacing mode timeintervals started in response to detecting asystole. In some cases, anotification is generated each time asystole is detected. An asystolenotification may be generated immediately following an asystole episodedetection (or after a threshold number of asystole episodes aredetected) and the notification may be immediately transmitted to anexternal device 40 or delivered to the patient or transmitted toexternal device 40 at a predetermined time of day.

In some examples, ICD 14 may be programmed to operate only in thesensing without pacing mode without automatic switching to a temporarypacing mode. For example, the physician may enable or disable theasystole backup pacing feature of the ICD while ICD 14 remains capableof detecting asystole episodes and storing asystole data. If asystolebackup pacing is not enabled, “No” branch of block 212, the ICD 14remains in the sensing without pacing mode and returns to block 204 tocontinue sensing R-waves and monitoring for asystole. No pacing isdelivered, however, asystole episodes are detected and asystole episodedata may be accumulated in memory for transmission to an external device40 (shown in FIG. 1A) for display to a user for confirming the asystoledetection(s) and managing patient therapies. As such, in some examples,ICD 14 is programmable in either a sensing without pacing mode withasystole backup pacing disabled or in a sensing without pacing mode withasystole backup pacing enabled.

When ICD 14 is programmed to operate in the sensing without pacing modewith asystole backup pacing enabled, control circuit 80 automaticallyswitches to the temporary pacing mode at block 214 in response to theasystole detection made at block 210. In some examples, control circuit80 starts a temporary pacing mode termination timer at block 216 tolimit the time period that ICD 14 operates in the temporary pacing mode.In this way, the pacing mode may be referred to as a “temporary” mode inthat after a predetermined time period or other termination condition issatisfied, the ICD 14 automatically switches from the temporary pacingmode back to the sensing without pacing mode.

Upon switching to the temporary pacing mode, the control circuit 80enables the therapy delivery circuit 84 to start a pacing escapeinterval after which the first pacing pulse is delivered as shown inFIG. 5. In other examples, therapy delivery circuit 84 may be controlledto immediately deliver a pacing pulse at block 218, and start the pacingescape interval at block 220 after immediately delivery a pacing pulse.As described above in conjunction with FIG. 5, the pacing escapeinterval may be an asystole backup rate interval set to a minimumavailable pacing rate interval, to a bradycardia pacing lower rateinterval, to a sensor-indicated rate interval for providing rateresponsive pacing to meet the patient's metabolic demand, or an intervalbased upon one or more RR intervals occurring prior to the asystoledetection.

At block 222, the control circuit 80 determines if a terminationcondition is detected. A termination condition may be detection of atachyarrhythmia episode by control circuit 80, e.g., a VT or VFdetection. If VT or VF is being detected and is programmed to be atermination condition, “Yes” branch of block 222, the control circuit 80automatically switches back to the sensing without pacing mode (returnto block 202). Another termination condition that may be detected atblock 222 is the expiration of the temporary pacing mode terminationtimer started at block 216. When the termination timer expires, thetemporary pacing mode is terminated, and the ICD automatically switchesback to the sensing without pacing mode at block 202. The terminationtimer may be set to a predetermined time period, e.g., 30 seconds, oneminute, two minutes, five minutes, 10 minutes or other predeterminedtime period.

In another example, a termination condition that may be detected atblock 222 may be a threshold number of sensed R-waves, consecutive ornon-consecutive. For example, if two to five R-waves (or anotherpredetermined number) are sensed before the pacing mode terminationtimer expires, control circuit 80 may automatically switch back to thesensing without pacing mode to promote an intrinsic heart rhythm. Assuch, upon starting the termination time interval at block 216, anR-wave sensed event counter included in control circuit 80 may also beenabled. Consecutively sensed R-waves may be counted and if a pacingpulse is delivered the counter may be reset to zero. If a thresholdnumber of consecutive R-waves, e.g., 3 to 5 R-waves, are sensed, thetemporary pacing mode may be terminated early. In other examples theR-waves do not need to be sensed consecutively so that the R-wave sensedevent counter is not reset to zero by a pacing pulse. In some instances,the counter is decremented in response to a delivered pacing pulse andincremented in response to a sensed R-wave. When the counter reaches athreshold value, the temporary pacing mode may be terminated.

In other examples, a user may manually terminate the temporary pacingmode by sending a command using external device 40 or by applying amagnet over ICD 14. If a magnet is detected by control circuit 80 or thecommand is received, the ICD 14 switches back to the sensing withoutpacing mode.

If no termination condition is detected at block 222, and the pacingescape interval started at block 220 has not expired, the controlcircuit 80 waits for the next sensed R-wave or expiration of the pacingescape interval, whichever comes first. If an R-wave is sensed at block226, the pacing escape interval is restarted at block 220. The pacingpulse scheduled to be delivered at the expiration of the pacing escapeinterval is cancelled. The pacing escape interval may be reset to anasystole backup pacing interval, a bradycardia pacing lower rateinterval, a rate responsive pacing interval, an interval based on RRintervals occurring prior to asystole detection, or to a hysteresisinterval, as described above. It is understood that a blanking intervalmay be set following a pacing pulse during which sensing circuit 86 doesnot generate an R-wave sensed event signal, or any R-wave sensed eventsignals that are generated during the blanking interval are ignored bycontrol circuit 80.

If a pacing escape interval expires, “Yes” branch of block 224, controlcircuit 80 controls therapy delivery circuit 84 to deliver a pacingpulse at block 228. The pacing pulse delivered at block 228 may bedelivered by a high voltage output circuit or a low voltage outputcircuit according to any of the techniques described in conjunction withFIG. 4 and in the above-incorporated references. After a deliveredpacing pulse, the pacing escape interval is restarted at block 220.

In some examples, a limited number of pacing pulses are delivered at theasystole backup rate interval, e.g., at an interval of 2 secondresulting in 30 pulses per minute, and then the pacing escape intervalis set to a hysteresis interval without requiring a sensed R-wave tostart the hysteresis interval. For example, the pacing escape intervalmay be set to 2 seconds according to the preceding example upon each ofa predetermined number of consecutive pacing pulses, e.g., five tothirty pacing pulses, then the control circuit 80 sets the pacing escapeinterval to a hysteresis pacing interval at block 220 that is longerthan the asystole backup pacing interval, e.g., up to 3 seconds.

In still other examples, the first pacing pulse delivered at block 218starts a pacing escape interval set to a hysteresis interval to allowintrinsic conduction to return after a single pacing pulse. If no R-waveis sensed, the next pacing pulse delivered at block 228 starts a pacingescape interval set to the asystole backup rate interval. The pacingescape interval may be set to the asystole backup rate interval for eachpacing pulse delivered (or to the hysteresis interval if an R-wave issensed or a predetermined number of pacing pulses have been delivered)until the termination time interval expires.

FIG. 7 is a timing diagram 300 of events during the temporary pacingmode started at block 214 of FIG. 6 according to one example. Pacingpulse 302 is delivered after switching to the temporary pacing mode,which may be after expiration of a first pacing escape time intervalthat was started immediately upon switching to the temporary pacing modeor immediately after switching to the temporary pacing mode. Controlcircuit 80 sets a pacing escape interval timer to a hysteresis interval304 upon delivery of the first pacing pulse 302. A termination timer(not shown in FIG. 7) may also be started upon switching to thetemporary pacing mode. The initial hysteresis interval 304 promotesintrinsic conduction over a paced rhythm. If the sensing circuit 86 doesnot sense an R-wave during the hysteresis interval 304, however, thesecond pacing pulse 306 is delivered. Upon delivering the second pacingpulse 306, control circuit 80 sets the pacing escape interval timer toan asystole backup pacing rate interval 308, which may be a minimumavailable rate interval, a programmed lower rate interval, or asensor-indicated pacing rate interval if the temporary pacing mode is arate responsive mode.

In some examples, all subsequent pacing pulses, e.g., pacing pulse 310,are delivered at the asystole backup rate interval 308 following animmediately preceding pacing pulse, e.g., pacing pulse 306, or apreceding R-wave sensed by the sensing circuit 86, until the terminationtimer expires or another temporary pacing mode termination condition isdetected, whichever comes first. In other examples, if an R-wave sensedevent signal 314 is produced by the sensing circuit 86 during anasystole backup rate interval 312, control circuit 80 sets the pacingescape interval timer to the hysteresis interval 316 to allow intrinsicconduction to occur. The hysteresis intervals 304 and 316 are longerthan the asystole backup rate interval 308. Hysteresis intervals 304 and316 are shown to be equal but the initial hysteresis interval 304 may bedifferent than the hysteresis interval 316 set in response to R-wavesensed event signal Rs 314. The pacing pulse scheduled to occur at theexpiration of the asystole backup rate interval 312 is withheld. If thehysteresis interval 316 expires without an R-wave sensed by sensingcircuit 86, a pacing pulse 318 is delivered. The pacing escape intervaltimer may be reset to the asystole backup rate interval upon delivery ofpacing pulse 318. If another R-wave is sensed during hysteresis interval316, the pacing escape interval timer is reset to the hysteresisinterval.

FIG. 8 is another timing diagram 400 of events during a temporary pacingmode started in response to detecting asystole during the sensingwithout pacing mode according to another example. In this example, thepacing escape interval timer is set to a first pacing escape interval404 upon switching to the temporary pacing mode at 402. The first pacingpulse 405 is delivered upon expiration of the first pacing escapeinterval 404. Pacing interval 404 may be a shorter interval than anasystole backup pacing rate interval 408 to provide an initial pacingrate faster than the programmed asystole backup pacing rate. Aninitially faster rate may provide hemodynamic support needed after along asystolic interval. While a single pacing pulse 405 is showndelivered at the initial rate interval 404, a series of two or morepacing pulses may be delivered at the initial pacing escape interval404. For example, a predetermined number such as 2, 3, 5 or otherpredetermined number of pacing pulses may be delivered at the initialrate interval 404. The initial rate interval 404 may be 50 to 1000 msshorter than the programmed asystole backup pacing rate interval 408. Inone instance, if the asystole backup pacing rate interval is twoseconds, to provide a backup asystole pacing rate of 30 pulses perminute, the initial pacing rate interval 404 may be one second toprovide one or more heart beats at a rate of 60 beats per minute. Inother examples, the first pacing escape interval 404 may be determinedby control circuit 80 as a sensor-indicated pacing rate based on apatient activity signal received from sensor 92.

After a predetermined number of pacing pulses 405 delivered at theinitial pacing escape interval 404, one or more subsequent pacing pulses406, 407, and 410 are scheduled and delivered at the asystole backupinterval 408 (in the absence of R-wave sensed event signals from sensingcircuit 86). Control circuit 80 may include a counter for counting thepacing pulses 405, 406, 407 and 410 delivered during the temporarypacing mode. In some examples, all delivered pacing pulses are countedby incrementing the counter each time a pacing pulse is delivered. Inother examples, the counter is reset to zero or decremented when anR-wave is sensed by sensing circuit 86.

Control circuit 80 compares the counter to a predetermined number ofpacing pulses. If the predetermined number of pacing pulses is reached,the pacing escape interval timer is set to a hysteresis interval 412longer than the asystole backup rate interval 408 upon delivering thelast pacing pulse 410 of the predetermined number of pacing pulses. Bysetting a hysteresis interval 412 periodically during the pacing mode,intrinsic conduction may be allowed to return to minimize pacing. If thesensing circuit 86 does not sense an R-wave during the hysteresisinterval 412, a pacing pulse 414 is delivered, and the control circuit80 may return to setting the pacing escape interval timer to theasystole backup rate interval 408 for another predetermined number ofpacing pulses (or until a termination condition is detected).

A total number of pacing pulses delivered during the temporary pacingmode may be counted by control circuit 80 and included in asystoleepisode data stored in memory 82 along with a cardiac electrical signalsegment. A histogram of pacing intervals, e.g., intervals 304, 308, 312,and 316 in FIG. 7 or intervals 404, 408 and 412 in FIG. 8, occurringduring the temporary pacing mode may also be stored with the asystoleepisode data for transmission to external device 40.

FIG. 9 is a flow chart 500 of a method for controlling a response todetecting asystole by ICD 14 according to another example. At block 502,the ICD is operating in the sensing without pacing mode. Upon initiatingthe sensing without pacing mode, control circuit 80 starts asystoletimers, which may include a probable asystole timer and an asystoledetection timer as shown in FIG. 5. If control circuit 80 receives anR-wave sensed event signal (block 505), the asystole timers arerestarted at block 506. If the probable asystole timer expires at block508, control circuit 80 enables recording of a cardiac signal episode inmemory 82 at block 510. As described in conjunction with FIG. 5, aprobable asystole timer may be set to a shorter time interval than theasystole detection timer so that a recording of the cardiac electricalsignal may be obtained leading up to asystole detection and prior tostarting asystole backup pacing.

If the asystole detection timer has not expired (block 512) and anR-wave sensed event signal is received by control circuit 80, “yes”branch of block 514, the asystole timers are restarted at block 506. Insome examples, if the probable asystole timer expires but the asystoledetection timer does not, the episode recording started at block 520 maybe terminated and may be cleared from memory 82 or overwritten the nexttime the probable asystole timer expires. If the asystole detectiontimer does expire before an R-wave sensed event signal is received,“yes” branch of block 512, control circuit 80 detects asystole at block516. Although in the example described in FIG. 9 two separate timers areutilized, a probable asystole timer and an asystole detection timer,only a single timer may be utilized as described in blocks 202, 203,204, 206, and 208 of FIG. 6 in place of the two timer techniquedescribed above.

In some examples, an asystole detection counter may be increased atblock 518 in response to detecting the asystole. The asystole detectioncounter may be increased each time an asystole is detected and may bereset upon an interrogation command, upon user command or upon switchingfrom a permanent bradycardia pacing mode to the sensing without pacingmode. At block 520, the asystole counter is compared to a threshold forswitching from the sensing without pacing mode to a permanentbradycardia pacing mode.

If the counter has not reached the threshold for switching to apermanent bradycardia pacing mode at block 520, and asystole backuppacing is not enabled, “no” branch of block 526, the control circuit 80stores the asystole episode data at block 544 and continues to monitorfor asystole episodes in the sensing without pacing mode by returning toblock 502. Asystole episodes may continue to be detected during thesensing without pacing mode for the useful purpose of collecting andstoring cardiac electrical signal data of the asystole episode(s)(enabled at block 510) for transmission to an external device 40 forreview by a physician. Asystole episode data recorded at block 544 mayinclude a cardiac electrical signal segment, RR interval data precedingthe asystole detection, patient activity data, patient posture data, orother data, e.g., as described in conjunction with block 211 of FIG. 6.The asystole episode data obtained and recorded by ICD 14 in theambulatory patient provides useful information to a clinician for makingdiagnostic and therapy management decisions. Such recordings ofspontaneous asystole episodes may not be obtainable in a clinic oroffice visit setting or, if obtained, may represent different clinicalconditions that are not necessarily representative of spontaneousasystolic events occurring outside a clinical setting.

If the counter has reached a threshold number of asystole detections atblock 520, the ICD 14 may automatically switch to a permanentbradycardia pacing mode at block 524 depending on whether switching topermanent bradycardia pacing is enabled or disabled. ICD 14 may beprogrammably enabled to operate in the sensing without pacing mode only,with both automatic switching to a temporary pacing mode for asystolebackup pacing support disabled and automatic switching to a permanentbradycardia pacing mode disabled. In other cases, the ICD 14 may beprogrammed to operate in the sensing without pacing mode with automaticswitching to the temporary, asystole backup pacing mode enabled but withautomatic switching to a permanent bradycardia pacing mode disabled. Instill other cases, ICD 14 may be programmed to operate in the sensingwithout pacing mode with both automatic switching to a temporary pacingmode enabled and automatic switching to a permanent bradycardia pacingmode enabled.

Depending on the pacing capture threshold required for pacing thepatient's heart using extra-cardiovascular electrodes, and the patient'stolerance for extra-cardiovascular pacing, a clinician may or may notenable ICD 14 to automatically switch to a permanent bradycardia pacingmode. In some cases, skeletal muscle stimulation or other unintendedstimulation caused by the extra-cardiovascular pacing may be poorlytolerated or painful to a patient. Accordingly, multiple programmablebradycardia therapy operating mode options may be available includingsensing without pacing only; sensing without pacing and automaticswitching to temporary asystole backup pacing enabled; sensing withoutpacing with automatic switching to temporary asystole backup pacingdisabled and automatic switching to permanent bradycardia pacingenabled; and sensing without pacing with both automatic switching totemporary asystole backup pacing and automatic switching to permanentbradycardia pacing enabled. As used herein, a temporary pacing moderefers to a pacing mode that switches back to the sensing without pacingmode after a termination condition is detected as described previouslyherein. A “permanent pacing mode” refers to a pacing mode that remainsin effect until the pacing mode is reprogrammed by a user, e.g., usingexternal device 40, without automatic switching back to the sensingwithout pacing mode. In some cases, however, a permanent pacing mode maybe automatically switched back to the sensing without pacing mode toconserve longevity of power source 98 if a pacing pulse has not beendelivered for a predetermined time period, e.g., for 24 hours, threedays, one week or other predetermined time period.

If the asystole detection counter reaches the threshold at block 520,and automatic switching to permanent bradycardia pacing is enabled, asdetermined at block 522, ICD 14 automatically switches to a permanentbradycardia pacing mode at block 524. During the permanent bradycardiapacing mode, bradycardia pacing is delivered according to a programmedlower pacing rate and bradycardia pacing mode, such as VVI, VVI(R), VVTor VVT(R) and other pacing control parameters. The pacing lower rate maybe adjusted to a sensor-indicated pacing rate when rate-responsivepacing is enabled, such as activity sensor based, rate-responsivepacing.

If the counter has not reached the threshold at block 520, or if it hasand automatic switching to a permanent bradycardia pacing mode is notenabled but asystole backup pacing is enabled, “yes” branch of block526, the control circuit 80 starts the temporary asystole backup pacingmode at block 530. In some examples, control circuit 80 may determineone or more rest conditions at block 532 upon switching to the temporaryasystole backup pacing mode. A rest condition may be any of the time ofday, patient activity and/or patient posture determined at block 532.Patient activity and/or patient posture may be determined by controlcircuit 80 based on a signal received from the activity and posturesensor 92. Control circuit 80 may include a clock or 24-hour timerinitialized to a real time setting for determining time of day. The restcondition(s) may be determined from signals received by control circuit80 from activity/posture sensor 92 prior to the asystole detection andprior to the onset of the asystole episode, e.g., from activity andposture signals obtained over a time interval prior to the R-wave thatstarts the asystole episode.

At block 534, control circuit 80 determines whether the patient is in aresting state based on whether or not one or more rest conditions aresatisfied. For example, if activity and/or posture is/are determined atblock 532, one rest condition may be a low level of activity and anotherrest condition may be a non-upright posture. Another rest condition maybe time of day. If the time of day is nighttime or a programmable timeperiod that the patient is known to typically be resting, a rest stateis detected at block 534. A rest state may be detected based solely ontime of day based on a clock signal included in control circuit 80 orany combination of patient activity, patient posture, and/or time ofday. If a rest state is detected, the asystole backup pacing iscontrolled according to programmed resting state asystole backup pacingcontrol parameters at block 538.

A first set of control parameters may be stored in memory 82 forcontrolling the temporary asystole backup pacing mode during a detectedrest state, and a second set of control parameters may be stored forcontrolling the temporary asystole backup pacing mode when a rest stateis not detected. Some patients may experience long ventricular pauses orvery slow ventricular rates during a resting state, for example at nightwhile the patient is asleep. Since an extra-cardiovascular pacing pulsethat successfully captures the heart may also capture skeletal muscle orother non-cardiac tissue in some patients, the patient may perceive thepacing pulse, which could disrupt sleep. Accordingly, in some cases,resting state control parameters may include a longer asystole backuprate interval for controlling the pacing escape interval than thenon-resting state control parameters. A resting state asystole backuppacing rate interval may be greater than or equal to two seconds, threeseconds, four seconds or more and a non-resting state asystole backuppacing rate interval may be between one and two seconds, for example. Inother examples, asystole backup pacing may be withheld during theresting state so that no extra-cardiovascular pacing pulses aredelivered during the temporary pacing mode when a resting state isdetected based on one or more rest conditions.

In still other examples, other pacing control parameters may be setdifferently for controlling asystole backup pacing during a detectedresting state than during a non-resting state. For example a lower pulsevoltage amplitude, a lower pulse voltage amplitude and longer pulsewidth, or a lower pacing pulse amplitude safety margin or pulse widthsafety margin may be used to reduce the likelihood of sleep disturbanceduring the resting state due to extra-cardiac stimulation. In oneinstance, the pacing pulse amplitude is set to zero when a resting stateis detected.

Other control parameters that may be different during the resting statethan during a non-resting state may relate to the duration and number ofpulses being delivered. For example, a shorter termination time intervalmay be applied to terminate the temporary asystole backup pacing modeearlier during rest than during non-rest. Other termination conditions,such as a maximum number of pacing pulses may be reduced to reduce thetotal number of pulses delivered. The number of pulses delivered at theasystole backup pacing rate interval before a hysteresis interval (e.g.,interval 412 in FIG. 8) is scheduled may be reduced in order to allow anearlier return to intrinsic rhythm and/or a longer hysteresis intervalmay be used after a threshold number of pulses is delivered as shown inFIG. 8 or after sensing an R-wave as shown in FIG. 7.

The non-resting state asystole backup pacing control parameters mayinclude an initial, first pacing escape interval for controllingdelivery of one or more pulses at an initial, accelerated rate, such asthe first pacing escape interval 404 that is shorter than the asystolebackup pacing rate interval 408 as shown in FIG. 8. This feature of aninitial shorter pacing escape interval may be disabled during restingstate asystole backup pacing. In general, the resting state controlparameters may be set to less intensive settings to reduce the energy,number and/or rate of pacing pulses delivered during the temporaryasystole backup pacing mode when a resting state is detected.

The resting state asystole backup pacing is delivered at block 538according to programmed resting state control parameters. If a restingstate is not detected at block 534, the asystole backup pacing isdelivered at block 536 according to the non-resting state asystolebackup pacing control parameters.

During the temporary asystole backup pacing mode, including bothnon-resting state asystole backup pacing delivered at block 536 andresting state asystole backup pacing if delivered at block 538, thecontrol circuit 80 may verify capture at block 540. Capture of one ormore delivered pacing pulses may be verified, for example, based onreceiving an R-wave sensed event signal from sensing circuit 86 within acapture verification time interval following delivery of a pacing pulse.In some cases, if capture is not initially verified, the pacing output,either the pacing pulse voltage amplitude and/or the pacing pulse width,may be increased until capture is verified at block 540.

When a termination condition is detected at block 542, asystole episodedata is stored at block 544 along with the cardiac electrical signalrecording that was enabled at block 510. The asystole episode data mayinclude data relating to patient activity, patient posture, time of day,rest conditions used to detect a resting state, whether or not a restingstate was detected, pacing pulse amplitude and/or width, captureverification data that may include the pacing pulse amplitude and/orpulse width at which capture was verified, number of pulses delivered,number of pulses delivered at respective pacing pulse intervals, thetermination condition detected, the duration of the temporary asystolebackup pacing mode, the duration of the asystole episode, and RRinterval data leading up to the asystole episode. After termination ofthe temporary asystole backup pacing mode, control circuit 80 returns tothe sensing without pacing mode at block 502.

FIG. 10 is a flow chart 600 of a method for controlling automaticswitching to a temporary asystole backup pacing mode by ICD 14 accordingto another example. While the flow chart 500 of FIG. 9 shows detectionof a resting state being performed after switching to the temporaryasystole backup pacing mode, it is to be understood that operationsrepresented by the flow chart 500 and other flow charts presented hereinmay be performed in a different order or combination than the specificorder and combination shown by the illustrative examples. For instance,if asystole backup pacing is to be entirely withheld during a detectedresting state, operations performed at blocks 532 and 534 fordetermining rest conditions and detecting a resting state may bedetermined during the sensing without pacing mode for withholdingautomatic switching to the temporary asystole backup pacing mode.

As shown in FIG. 10, during the sensing without pacing mode in effect atblock 602, the rest condition(s) may be determined at block 604 prior tostarting asystole timer(s) or detecting asystole. Control circuit 80 mayset asystole detection criteria to a first set of criteria at block 608when a resting state is detected (“yes” branch of block 606) and to asecond set of asystole detection criteria at block 610 when a restingstate is not detected (“no” branch of block 606). For example, theasystole detection timer may be to a first asystole detection timeinterval during a detected resting state and to a second asystoledetection time interval when a resting state is not detected. The firstasystole detection time interval may be a relatively longer timeinterval, e.g., at least four seconds or longer, and the second asystoledetection timer interval may be relatively shorter than the firstasystole detection time interval, e.g., at least two seconds but lessthan the first asystole detection time interval.

In this way asystole detection during a detected resting state mayrequire a longer ventricular pause to detect asystole than during anon-resting state. A relatively longer ventricular pause may betolerated by the patient during rest without requiring asystole backuppacing. During a non-resting state, however, a relatively longerventricular pause may lead to hemodynamic insufficiency or physicalsymptoms justifying a relatively shorter asystole detection timeinterval during a non-resting state than during a resting state.

If a probable asystole timer is set, as described in conjunction withFIG. 9, the probable asystole timer may be set to the same time intervalat blocks 608 and 610 so that triggering of cardiac electrical signalstorage occurs after the same time interval during both resting andnon-resting states. In other examples, the probable asystole timer maybe set to a longer time interval during the resting state than duringthe non-resting state.

At block 612, asystole is detected according to one of the detectioncriteria set at either block 608 or 610. At block 614, control circuit80 may determine whether a resting state was detected at block 606 ordetermine whether a resting state is now detected based on differentrest condition criteria than the criteria used at block 606. Forinstance, a resting state may be detected at block 606 based only ontime of day such that different asystole detection criteria are used atnight than during the day. Detection of a resting state at block 614,however, may additionally or alternatively require the patient activityand/or patient posture meet a rest condition in order to detect aresting state. If a resting state is detected based on one or moredetermined rest conditions at block 614, or based on the prior detectionof a resting state made at block 606, control circuit 80 may withholdautomatic switching to the temporary asystole backup pacing mode byreturning to block 608. The automatic switching to the temporaryasystole backup pacing mode at block 530 may be withheld to preventasystole backup pacing while the patient is determined to be in aresting state. ICD 14 remains in the sensing without pacing mode, untilan asystole detection is made and a resting state is not detected.

If a resting state is not detected at block 614, and asystole backuppacing is enabled at block 616, control circuit 80 automaticallyswitches to the temporary asystole backup pacing mode at block 620. Ifasystole backup pacing is not enabled, “no” branch of block 616, thecontrol circuit 80 remains in the sensing without pacing mode andreturns to block 610 to continue utilizing the non-rest asystoledetection criteria based on the resting state not being detected atblock 614. In other examples, if a different set of resting stateasystole detection criteria is used at block 606 than at block 614, thecontrol circuit 80 may return to block 606 for determining if a restingstate exists according to a first set of resting state detectioncriteria after block 614 (“yes” branch) and after block 616 (“no”branch). A first set of resting state detection criteria used at block606, e.g., time of day only, may be different than a second set ofresting state detection criteria used at block 614, e.g., patientactivity and/or patient posture in addition to or alternatively to timeof day. These different sets of resting state detection criteria allowautomatic switching to the temporary asystole backup pacing mode in theevent of asystole detection occurring at night according to restingstate asystole detection criteria, when the patient is typically atrest, but the patient happens to be upright and/or active and the needfor asystole backup pacing is properly indicated.

FIG. 11 is a conceptual diagram of an ICD system that may performasystole detection and provide an asystole response according to anotherexample. Techniques disclosed herein have been described in conjunctionwith an ICD system including an implantable medical lead carryingextra-cardiovascular electrodes, but aspects of these techniques may beutilized in conjunction with other cardiac electrical sensing lead andelectrode systems. For example, the techniques for detecting asystoleand providing a response to detecting the asystole as described inconjunction with the accompanying drawings may be implemented in anyimplantable or external medical device enabled for sensing cardiacelectrical signals, including implantable pacemakers, ICDs or cardiacmonitors coupled to transvenous or epicardial leads carrying sensingelectrodes; leadless pacemakers, ICDS or cardiac monitors havinghousing-based sensing electrodes; and external pacemakers,defibrillators, or cardiac monitors coupled to external, surface or skinelectrodes.

For example, system 10′ shown in FIG. 11 may include ICD 14 coupled toextra-cardiovascular lead 16 as described in conjunction with FIGS.1A-2C above and may further include an intra-cardiac pacemaker 50.Examples of ICD systems including an intracardiac pacemaker or pacingpulse delivery device and an ICD coupled to an extra-cardiovascular leadare generally disclosed in U.S. Pat. No. 9,168,380 (Greenhut, et al.)and in U.S. patent application Ser. No. 14/823,405 (Sharma, et al.).These systems including an intracardiac pulse delivery device may beconfigured to perform the techniques disclosed herein for detectingasystole and providing an asystole response, which may include storingasystole episode data, switching from a sensing without pacing mode to atemporary backup pacing mode and/or switching to a permanent bradycardiapacing mode. Detection of asystole may be performed by the ICD 14 and/orby the intracardiac pacemaker 50 in the example system 10′. Intracardiacpacemaker 50 may perform the automatic switching to the temporary orpermanent pacing mode automatically or in response to a signal from ICD14 for delivering the asystole backup pacing, e.g., as described inconjunction with FIGS. 5A, 5B, 7 and 8.

Thus, a method and apparatus for detecting and responding to asystole inan ICD have been presented in the foregoing description with referenceto specific embodiments. In other examples, various methods describedherein may include steps performed in a different order or combinationthan the illustrative examples shown and described herein. It isappreciated that various modifications to the referenced embodiments maybe made without departing from the scope of the disclosure and thefollowing claims.

1. An implantable cardioverter defibrillator (ICD) system, comprising: asensing circuit configured to receive a cardiac electrical signal via asensing electrode vector and sense cardiac events from the cardiacelectrical signal; a therapy delivery circuit configured to deliverelectrical pacing pulses to a heart of a patient via a pacing electrodevector; and a control circuit coupled to the sensing circuit and thetherapy delivery circuit and configured to automatically switch betweena sensing without pacing mode and a temporary pacing mode, and furtherconfigured to: detect asystole based on the cardiac electrical signalwhile operating in the sensing without pacing mode, in response todetecting the asystole, determine if asystole backup pacing is enabled,and automatically switch to the temporary pacing mode in response to theasystole backup pacing being enabled.
 2. The ICD system of claim 1,wherein the control circuit is configured to remain in the sensingwithout pacing mode in response to the asystole backup pacing mode notbeing enabled.
 3. The ICD system of claim 1, further comprising: amemory coupled to the control circuit; and a telemetry circuitconfigured for bi-directional wireless communication with an externaldevice, wherein the control circuit is configured to store a segment ofthe cardiac electrical signal in the memory in response to detecting theasystole, and control the telemetry circuit to transmit the storedcardiac electrical signal segment to the external device.
 4. The ICDsystem of claim 1, further comprising a memory coupled to the controlcircuit, wherein the control circuit is configured to: set a firstasystole time interval in response to an R-wave sensed by the sensingcircuit; set a second asystole time interval in response to the sensedR-wave; enable recording of a segment of the cardiac electrical signalin the memory in response to the first asystole time interval expiringwithout sensing a next R-wave by the sensing circuit during the firstasystole time interval; and detect the asystole in response to thesecond asystole time interval expiring without sensing a next R-wave bythe sensing circuit during the second asystole time interval.
 5. The ICDsystem of claim 4, wherein the second time interval is at least twoseconds and the first time interval is shorter than the second timeinterval.
 6. The ICD system of claim 1, wherein the control circuit isconfigured to: detect a termination condition; and switch from thetemporary pacing mode back to the sensing without pacing mode upondetection of the termination condition.
 7. The ICD system of claim 6,wherein: the ICD further includes a telemetry circuit configured forbi-directional wireless communication with an external device; and thecontrol circuit is configured to detect the termination condition bydetecting one of: an expiration of a predetermined termination timeinterval; a tachyarrhythmia episode; a magnet applied over the ICD,receipt of a termination command by the telemetry circuit, apredetermined number of pacing pulses delivered during the temporarypacing mode, or a predetermined number of sensed R-waves during thetemporary pacing mode.
 8. The ICD system of claim 1, wherein the controlcircuit is configured to control the therapy delivery circuit to: set apacing escape interval to a first value upon switching to the temporarypacing mode; deliver up to a predetermined number of pacing pulses at arate corresponding to the first value when an R-wave is not sensed bythe sensing circuit during the pacing escape interval set to the firstvalue; set the pacing escape interval to a second value longer than thefirst value after a last one of the predetermined number of pacingpulses is delivered at the rate corresponding to the first value; anddeliver a subsequent pacing pulse at a rate corresponding to the secondvalue when an R-wave is not sensed during the pacing escape interval setto the second value.
 9. The ICD system of claim 1, wherein the controlcircuit is configured to control the therapy delivery circuit to: set apacing escape interval to a first value upon switching to the temporarypacing mode; deliver up to a first predetermined number of first pacingpulses at a first rate corresponding to the first value of the pacingescape interval when an R-wave is not sensed by the sensing circuitduring the pacing escape interval set to the first value; set the pacingescape interval to a second value longer than the first value after thefirst predetermined number of pacing pulses are delivered at the firstrate; deliver up to a second predetermined number of second pacingpulses at a second rate corresponding to the second value of the pacingescape interval when an R-wave is not sensed during the pacing escapeinterval set to the second value; set the pacing escape interval to athird value that is longer than the second value upon delivering a lastone of the second predetermined number of second pacing pulses; inresponse to the pacing escape interval set to the third value expiringwithout an R-wave sensed by the sensing circuit, deliver a third pacingpulse upon expiration of the pacing escape interval set to the thirdvalue; and set the pacing escape interval to the second value inresponse to delivering the third pacing pulse.
 10. The ICD system ofclaim 1, wherein the control circuit is configured to: determine atleast one rest condition; determine whether the patient is in one of arest state or a non-rest state based on the at least one rest condition;in response to determining the patient being in the rest state, controlthe therapy delivery circuit to deliver asystole backup pacing accordingto a first setting of a pacing control parameter; and in response todetermining the patient is in the non-rest state, control the therapydelivery circuit to deliver asystole backup pacing according to a secondsetting of the pacing control parameter different than the firstsetting.
 11. The ICD system of claim 10, wherein the ICD furthercomprises a sensor producing a signal correlated to at least one ofpatient activity or patient posture; the control circuit beingconfigured to determine the at least one rest condition by determiningat least one of a time of day, a patient activity, or a patient posture.12. The ICD system of claim 10, wherein the pacing control parameter isone of a pacing escape interval, a predetermined number of pacingpulses, a pacing pulse amplitude, a hysteresis interval, or a temporarypacing mode termination condition.
 13. The ICD system of claim 1,wherein the control circuit is configured to: detect a rest state of thepatient during the sensing without pacing mode; and in response todetecting the rest state, perform at least one of: set an asystoledetection timer to a first time interval that is longer than a secondasystole detection timer used to detect asystole when a rest state isnot detected; or withhold switching to the temporary asystole backuppacing mode.
 14. The ICD system of claim 1, wherein the control circuitis configured to: increase an asystole episode counter in response todetecting the asystole; compare the asystole episode counter to apermanent pacing mode switch threshold; in response to the permanentpacing mode switch threshold being reached, determine if switching to apermanent pacing mode is enabled; switch from the sensing without pacingmode to the permanent pacing mode in response to switching to thepermanent pacing mode being enabled; and control the therapy deliverycircuit to deliver cardiac pacing according to the permanent pacingmode.
 15. The ICD system of claim 1, further comprising at least one of:a patient notification circuit configured to generate a notificationsignal perceivable by the patient; or a telemetry circuit configured totransmit a notification signal to an external device, wherein thecontrol circuit is configured to control at least one of the patientnotification circuit or the telemetry circuit to generate a notificationin response to detecting the asystole episode.
 16. The ICD system ofclaim 1, further comprising: a memory coupled to the control circuit;and a telemetry circuit configured for bi-directional wirelesscommunication with an external device, wherein the control circuit isconfigured to store asystole episode data comprising a segment of thecardiac electrical signal in the memory with a date and time stamp inresponse to detecting the asystole, and control the telemetry circuit totransmit the asystole episode data to the external device.
 17. The ICDsystem of claim 16, wherein the control circuit is further configured tostore the asystole episode data by: determining and storing at least oneof: a total number of pacing pulses delivered during the temporarypacing mode; a histogram of pacing intervals during the temporary pacingmode; or pacing capture verification data determined by verifyingcapture of at least one pacing pulse delivered during the temporarypacing mode.
 18. The ICD system of claim 16, further comprising a sensorproducing a signal correlated to at least one of patient activity orpatient posture; wherein the control circuit is further configured tostore the asystole episode data by determining a patient rest conditionfrom the sensor signal and storing the rest condition with the asystoleepisode data.
 19. The ICD system of claim 1, further comprising anextra-cardiovascular lead configured to be coupled to the ICD, theextra-cardiovascular lead carrying at least one electrode of theextra-cardiovascular pacing electrode vector.
 20. The ICD system ofclaim 1, wherein the control circuit is further configured to: triggerthe therapy delivery circuit to deliver a pacing pulse in response to acardiac event being sensed by the sensing circuit after switching to thetemporary pacing mode, the pacing pulse being delivered by the therapydelivery circuit within a physiological refractory period of the sensedcardiac event.